I'm sure it's slower than light-speed, or rather slower to activate and complete than light-speed. While Apparation may be instantaneous as far as the user is concerned, it has to be consciously activated, meaning you have to think about your destination before-hand and thus it can only act as fast as you can react.
One case in point is Dobby and Bellatrix Lestrange at Malfoy Manor. Dobby used Apparition to escape with several hostages but Bellatrix threw a knife at them as they were Apparating away. The knife stuck Dobby in the chest, causing his death upon his arrival.
Using this clip we can see that the knife was thrown before Dobby began Apparating and was transported along with Dobby and the hostages before they arrived:
This means that the knife reached Dobby before Apparation was completed so either the knife traveled instantaneously as well, or it was thrown at a speed faster than Dobby could Apparate. There's no evidence that Bellatrix used magic on the knife to make it travel faster, and we can clearly see it moving in flight. Thus, it can't be moving instantaneously, and thus neither can Apparation. On average, thrown knives travel between 26 and 30 mph, or 42-48 kph, and as there's no evidence that Bellatrix has superhuman strength or speed, this seems like a good speed range for the knife to travel at.
If Apparation was instantaneous then the knife would not have killed Dobby. Thus, it's probably safe to conclude that Apparation requires at least a second or two to be completed, as human reaction speed averages around 200 microseconds.